As is known, air filters generally comprise a rigid external casing, normally called a filter box, which is provided with an inlet for the air to be filtered and an outlet for the filtered air.
On-board a vehicle, the filter box is normally located internally of the engine compartment, where it is associated to a first conduit destined to connect the air inlet with an air intake afforded in the bonnet or in the front part of the vehicle, and to a second conduit destined to connect the air outlet with the aspiration manifold of the engine.
A filter cartridge is housed in the filter box, destined to filter the air moving from the inlet towards the outlet.
The filter cartridge must have a good filtering efficiency level without causing excessive load loss, and has to have quite a high storage capacity, in order not to have to be replaced too frequently.
The filter cartridge can be of various constructional types, depending on geometric and dimensional characteristics of the filter box.
In particular, the prior art comprises filter cartridges having filter walls conformed as rectangular sheets or toroidal bodies, possibly pleated in order to increase the filtering surface and the storage capacity.
These filter cartridges are rather large and therefore have the drawback of having to be housed internally of unwieldy filter boxes, which take up much space internally of the engine compartment.
Replacement of these filter cartridges further requires large manoeuvring space internally of the engine compartment, which space must be left empty ready for the manoeuvring.
Also known are filter cartridges which comprise a tubular and slightly conical filter wall, which are destined to be directly inserted internally of the aspiration conduit of the engine.
In this case the filter box is constituted by a tract of the aspiration conduit.
These tubular filter walls can be realised by a non-woven textile felt or a more traditional pleated wall.
The tubular filter walls have the drawback of increasing load loss, due to the small space between the wall and the conduit in which it is contained, and of having a storage capacity (and therefore a working life) which is rather limited due to the small filtering surface proffered.
Also known are multi-wall filter cartridges, which comprise a plurality of tubular filter walls which are arranged aligned in parallel rows, such as to form a bundle.
The tubular filter walls are closed at an end by a continuous-wall support flange, while at the other end they are supported by a holed support flange, which is provided with a plurality of through-holes that are singly aligned with the cavity of a respective filter wall.
Multi-wall filter cartridges are housed internally of a filter box, where the tubular filter walls are generally crossed in parallel by the air to be filtered.
On the basis of the embodiment, the air to be filtered can cross the tubular filter walls from the inside towards the outside, or from the outside towards the inside.
Multi-wall filter cartridges have the advantage of guaranteeing rather contained load loss.
A drawback is that multi-wall filter cartridges have the disadvantage of being overall rather voluminous, and thus need large filter boxes which occupy a lot of space internally of the engine compartment.
An aim of the present invention is to resolve the above-mentioned drawbacks in the prior art.
A further aim of the invention is to attain the above objective in the ambit of a simple, rational and relatively inexpensive solution.